The present invention relates to methods and apparatus for recording and playing back EGG signals and more particularly to such methods and apparatus in which the EGG signals are processed to compensate for variations in amplitude and phase of the EGG signals due to the recording process.
Prior art systems, known as Holter recorders and playback systems, record EGG signals which are provided from a patient via electrodes to a small recorder carried by the patient. The EGG signals are continuously recorded on an analog tape typically over a 24 hour period. Several prior art systems use a conventional C60 analog cassette tape of the type typically used to record audio information. In order to do so, the tape must be slowed down approximately 1/50th of normal audio speed. Usually three channels of EGG data are recorded plus a fourth channel which includes a timing signal that is generated by the recorder.
After recording, the tape is played back on a device which runs the tape at high speed and processes the analog signals derived from the tape. Such processing typically includes filters which are intended to compensate for dropoff at both the low and high frequencies and for phase distortion which results from the magnetic recording and playback process. The filtered signals are digitized and further processed into a report for analysis by a physician. Typically, the digitized data is transmitted to a personal computer connected to the playback system for further analysis.
The data transmitted to the personal computer, however, can exhibit phase and magnitude distortion due to non-linearities in the recording and playback systems. Moreover, the phase and magnitude distortion produced by a system for recording and playing back ECG signals can vary over time. It can vary as a result of wear on the heads for recording and playing back the signal. It can also vary in response to misalignment of the mechanical components which affect the record and playback head compliance with the tape. Independently of time, distortion can vary as a result, e.g., of using an analog tape having a different thickness or type of tape coating thereon.
Prior art techniques for calibrating the ECG recorder include recording a series of rectangular calibration pulses of known amplitude on each of three channels used the recording ECG signals. Pulses are played back through a playback deck and passed through analog compensation filters to restore the pulse's original phase and frequency content. The playback unit is calibrated by measuring the amplitude of the calibration pulses and thereafter generating a data scaler which is applied to the ECG data.
There are several problems with this approach. One is that phase and magnitude distortion can vary from channel to channel. Another problem results from a phenomenon called tape dropout. Tape dropout is an apparent reduced recording level on a tape caused by variations in the recording medium. When such areas are encountered during the calibration process, accuracy of calibration is adversely affected.
Accordingly, a need remains for a more accurate method of calibrating each ECG channel.